This disclosure relates generally to programmable process controllers and, more particularly, to safety input output devices used with programmable process controllers.
At least some known process controllers include special purpose computers, for example, programmable logic controllers (PLCs) that are used for the control of industrial machines and processes. During execution of a stored control program, the process controller reads inputs from the controlled process and, in accordance with the logic of the control program, provides outputs to the controlled process.
PLC's are normally constructed modularly with a processor and I/O circuitry that are constructed as separate modules that may be inserted in a chassis and connected together through a common backplane. The I/O modules permit connection of the input and output field wiring to the processor through the backplane. The field wiring typically includes multi-conductor cables that extend from the PLC cabinet to a termination panel proximate a machine or process to be controlled. The multi-conductor cable may extend for significant distances and may be broken into several terminated segments as it is routed through a plant.
Some machines and/or processes include safety functions intended to protect operators and/or equipment from injury or damage due to a malfunction or intrusion into a field of operation of the machine. Safety systems, for use in providing such safety functions are generally embodied in a separate qualified safety system or incorporated into the control system by safety qualifying the entire control system.
Such discrete safety systems can be costly to install and maintain, especially for complex control applications and qualifying the entire control system incurs significant expense of initial safety qualification and continued maintenance of the system safety qualification. Additionally, long distances between the controlled process or machine and the process controller, intervening signal conditioning and processing devices, and the scan rate of the process controller may delay the actuation of the safety function due to the response time of the system.